The 2.5" pipe down the hill is good for gravity feed (minimal pressure loss due to pipe friction.) The GPM if you opened the 2.5" pipe in that location does not matter unless you do that (and would be at 0 PSI.) The GPM will be the GPM of your sprinkler heads (+ any leaks, so don't leak.)
The 200 feet of 3/4" feed to the sprinklers is likely where the problem will be; much more pipe friction (dynamic head) so pressure when flowing will be lower or much lower. How much lower will depend on the flow rate of the chosen sprinklers. Larger pipe will reduce the dynamic head at any given flow. There are plenty of irrigation calculators to help with that on the web. Here's one with links to others. http://irrigation.wsu.edu/Content/Calculators/General/Pressure-Loss-With-Outlets.php
I get 28 PSI loss (so 12 PSI remaining of your 40) for 200 feet of 0.75" pipe (beware of bad results on that calculator if you enter 3/4 instead) with 4 outlets, and 2.5 GPM per head is pretty low for a lot of sprinklers, but you haven't given a number.
To get to "1 PSI loss in the pipe" as you've guessed, your total flow would be 1.65 GPM so 0.41 GPM per sprinkler head, which seems - unlikely.
You can sort what size lateral pipe you need by applying that calculator (if the outlets are equally spaced - if not, you'll have to get more complex with those calculatons) to your actual sprinkler head flow requirements.
Evidently the first sprinklers you mention have a range from 1.5-8.6 GPM.
4 @ 1.5 GPM = 6 GPM in 1.25" pipe gives just under 1 PSI loss with 4 outlets.
4 @ 8.6 GPM = 34.4 GPM requires 2.5 inch pipe to achieve similar dynamic head, which means you'll have to start accounting for additional head loss in your pipe down the hill at that rate. 2 inch pipe would only be 2.3 PSI loss in the lateral. 1.5" would be 9.5 PSI loss in the lateral and might or might not work for you, depending on minimum required pressure and how much additional you lose in the length of 2.5" pipe down the hill.
The minimum rated pressure (which also makes the circle covered smaller) for your brass ones is 30 PSI with 5.5GPM flow per head, so 22 GPM for four which you might just hit with 1.25" pipe at 10.09 PSI loss. 1.5 inch pipe would be safer at 4.15 PSI loss in the lateral. You'd want to examine the length (not vertical drop) of your 2.5" feed for additional losses, though they might be fairly low still (but I don't know how long that pipe is, as you haven't told us.)